Hard metal component with a graduated structure and methods of producing the component

ABSTRACT

A component is produced by powder metallurgy from hard metal. The alloy includes at least one grain growth-inhibiting additive from the group consisting of V, Cr, Ti, Ta and Nb with, at least locally, a graduated concentration profile. As a result, the mechanical properties also have a graduated profile. In the fabrication process, a dispersion or solution which contains the grain growth-inhibiting additive in finely distributed or dissolved form is applied to the surface of a green compact. Penetration of this dispersion or solution along open pores leads to a graduated distribution of the grain growth-inhibiting additive in the green compact. There is also described a process in which the grain growth-inhibiting additive in the form of a solution is distributed uniformly in the green compact and is then gradually broken down from edge regions by a heat treatment or a solvent.

BACKGROUND OF THE INVENTION

[0001] Field of the Invention

[0002] The invention lies in the metallurgy field. More specifically,the invention relates to a component produced by powder metallurgy froma hard metal alloy with a binder content of from 0.1 to 20% by weightwhich contains at least one grain growth-inhibiting additive. Theinvention, furthermore, pertains to a process for producing thecomponent.

[0003] The term hard metal is understood as meaning a composite materialwhich substantially comprises a carbidic component and a binder. Themost important carbidic components include the carbides or mixedcarbides of the metals W, Ti, Zr, Hf, V, Nb, Ta, Mo and Cr. Typicalbinder metals are Co, Ni and Fe. Additions of further hard materials,such as for example carbonitrides, are also used.

[0004] The properties of hard metals are determined by the ratio ofcarbide content to binder content, by the chemical composition, thecarbide grain size and the carbide grain size distribution. This opensup numerous options for matching the properties of hard metal to thecorresponding application area.

[0005] For example, increasing the binder content leads to animprovement in the fracture toughness and bending strength, combined, atthe same time, with a reduction in the hardness, rigidity andcompressive strength. A reduction in the carbide grain size leads to anincrease in the hardness, the compressive strength and the bendingstrength combined with a reduced impact toughness and fracturetoughness.

[0006] Nowadays, carbidic powders in the grain size range from 0.2 μm to15 μm are used for the production of hard metal components according tothe intended use. To reduce the grain coarsening during the sinteringoperation when fine-grained carbide powder is used, grain growthinhibitors are added. The most effective grain growth-inhibitingadditives are vanadium carbide, chromium carbide, titanium carbide,tantalum carbide and niobium carbide. In many cases, two or moreadditives are used, such as for example mixtures of VC and Cr₃C₂ or TaC,NbC and TiC. The grain growth-inhibiting additive can be distributedextremely finely in the main component as early as before or during thecarburizing step. However, it is also effective if the grain growthinhibitor is admixed with the hard metal powder or individualconstituents of the hard metal powder before, during or after milling.

[0007] Hard metal components may be subject to very differing localloads. Therefore, from a very early stage solutions which are based on amaterial composite comprising two or more hard metal alloys have beendiscovered and implemented. For example, U.S. Pat. No. 5,543,235describes a hard metal material composite which is produced by powdermetallurgy composite pressing, the individual material regions differingin terms of their composition or microstructure. A rotating compositetool which is composed of two hard metal alloys is also described inU.S. Pat. No. 6,511,265 B1 and international PCT publication WO01/43899. Production is likewise preferably effected by compositepressing.

[0008] A further process technique for the manufacture of a hard metalcomposite-body is described in U.S. Pat. No. 5,594,931. A surface layeror slip which consists of a powder mixture, a solvent, a binder and aplasticizer is applied to a green compact or core. The composite greencompact produced in this way is densified by sintering.

[0009] However, a drawback of the material composites described here isthat stress concentrations occur in the regions of the composite bodywhere materials with different properties meet one another. Furthermore,account must be taken of the fact that each material component has itsown sintering characteristics. This may cause distortion to thecomponent during sintering.

[0010] However, if the transition between two material regions is madewith a graduated composition, stress peaks can be substantially avoided.A structure of graduated composition is understood to mean that thecomposition changes gradually and continuously over a certain region.Especially in the case of coated hard metal, graduated formations in theregion of the layer, in the region of the layer/base material transitionand in the adjacent base material have long been known. This graduationis achieved, for example, by the addition of carbonitrides. During thesintering, the nitrogen in the edge zone of the hard metal body isbroken down. The metallic carbide-forming or nitride-forming elementsdiffuse toward the center of the hard metal body. This results in anincrease in the levels of binder in the region of the edge zone and agraduated transition to the matrix composition. For example, disposablecutting tool tips with a binder-rich edge zone adjacent to thehard-material layer have long been in use for the machining of steel.However, the graduation is restricted to a small region close to thesurface.

[0011] For high loaded components, it is advantageous to establish astructure which is graduated over a wide region. In this way, it ispossible to achieve considerable improvements to the service life,specifically if the mechanical demands imposed on the hard metal differin the edge region and the core region.

[0012] Since the usual binder metals, such as for example cobalt, have ahigh diffusivity at the sintering temperature, it is possible to achieveconcentration compensation in the transition zone between two hard metalalloys which have a differing cobalt content by means of diffusionprocesses. In this way, it is possible to establish a continuoustransition. A process for that purpose is described, for example, inU.S. Pat. No. 5,762,843 and European patent EP 0 871 556. A compositebody which comprises at least two regions which differ in terms of theirbinder content is produced by composite pressing. During the sintering,the temperature is to be set in such a way that the binder metaldiffuses out of the composite region with the higher binder content intothe composite region with a lower binder content. A drawback of thisprocess is that the sintering temperature has to be set very accuratelyin order not to produce complete concentration balancing and therebylose the different materials properties. A further drawback is thatcomposite pressing is associated with higher production costs than isthe case when a monolithic green compact is being produced. Europeanpatent applications EP 0 247 985 and EP 0 498 781 likewise describe hardmetal bodies with a binder phase gradient and a process for producingthem. In this case, first of all a sintered body with a uniformlydistributed η phase is produced by means of standard process steps usinga reduced-carbon starting powder mixture. A subsequent treatment in acarburizing atmosphere leads to partial dissolution of the η phase inthe region of the edge zone. The level of η phase increases graduallyand the binder content decreases gradually in the direction of thecenter of the hard metal body. However, a drawback is that the η phasehas an embrittling action. Moreover, the additional carburizing step istime-consuming and energy-consuming.

[0013] European patent application EP 0 111 600 describes a highlyloaded tool for rock drilling. The device comprises an inner region andan outer region, with a continuous transition of the mechanicalproperties between these regions. The proposed process technology is acomplex powder feed making it possible to continuously adjust the powderconcentration during the filling operation. A powder feed of this naturerequires complex apparatus and is difficult to control in terms ofprocess technology.

SUMMARY OF THE INVENTION

[0014] It is accordingly an object of the invention to provide a hardmetal component with a graduated structure and a manufacturing processwhich overcome the above-mentioned disadvantages of the heretofore-knowndevices and methods of this general type.

[0015] With the foregoing and other objects in view there is provided,in accordance with the invention, an article of manufacture made from ahard metal alloy, comprising:

[0016] at least one carbide, mixed carbide or carbonitride of the metalsselected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr,and V;

[0017] at least one grain growth-inhibiting additive selected from thegroup consisting of V, Cr, Ti, Ta, and Nb or a compound thereof, atleast one of said grain growth-inhibiting additives, at least locally,having a graduated concentration profile; and

[0018] at least one metallic binder selected from the group consistingof Co, Ni and Fe with a binder content of 0.1-20% by weight.

[0019] In accordance with an added feature of the invention, the hardmetal alloy has a graduated grain size profile, at least locally. Alsopreferably, the hard metal alloy has a graduated hardness profile.

[0020] In accordance with an additional feature of the invention, theconcentration of the grain growth-inhibiting additive decreasesgradually from the edge zone of the component toward the center of thecomponent. Conversely, the carbide grain size increases gradually fromthe edge zone of the component toward the center of the component.

[0021] In accordance with an alternative feature of the invention, theconcentration of the grain growth-inhibiting additive increasesgradually from the edge zone of the component toward the center of thecomponent. Conversely, the carbide grain size decreases gradually fromthe edge zone of the component toward the center of the component.

[0022] In accordance with a preferred embodiment, the graingrowth-inhibiting additive consists of Cr and/or V, or a compoundthereof. A maximum content of the grain growth-inhibiting additive,based on the hard metal alloy, is 2% by weight, and its contentdecreases gradually to a value x, where 0<x<1.0% by weight.

[0023] With the above and other objects in view there is also provided,in accordance with the invention, a method of producing the componentoutlined above. The method comprises the following steps:

[0024] producing a green compact from a hard metal alloy, containing atleast one carbide, mixed carbide or carbonitride of the metals selectedfrom the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V, atleast one metallic binder selected from the group consisting of Co, Ni,and Fe, an optional addition of wax or a plasticizer, with a standardpowder metallurgy compacting process or a standard shaping process;

[0025] producing a dispersion or solution containing at least one graingrowth-inhibiting additive selected from the group of metals consistingof V, Cr, Ti, Ta, and Nb or a compound thereof, in finely distributed ordissolved form;

[0026] applying the dispersion or solution to a surface of the greencompact, such as, for example, by dipping, spraying, and/or brushing;

[0027] targeted action to establish the concentration gradient; and

[0028] subjecting the article to heat consolidation.

[0029] In accordance with an alternative process, the manufacturecomprises the following steps:

[0030] producing a green compact from a hard metal alloy, containing atleast one carbide, mixed carbide or carbonitride of the metals selectedfrom the group consisting of W, Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V, atleast one metallic binder selected from the group consisting of Co, Ni,and Fe, an optional addition of wax or a plasticizer;

[0031] producing a solution containing at least one graingrowth-inhibiting additive selected from the group of metals consistingof V, Cr, Ti, Ta, and Nb or a compound thereof;

[0032] applying the solution to a surface of the green compact, forexample, by dipping, spraying, and/or brushing;

[0033] targeted action to establish the concentration gradient orcomplete infiltration;

[0034] gradually removing the grain growth inhibitor from regions closeto the surface by heat treating and/or with a solvent; and

[0035] subjecting the article to heat consolidation.

[0036] In other words, the above objects are achieved by a componentmade from a hard metal alloy and by a process for producing it, in whichthe hard metal alloy contains at least one carbide, mixed carbide orcarbonitride of the metals from the group consisting of W, Ti, Ta, Mo,Zr, Hf, V, Nb, Cr and V, at least one grain growth-inhibiting additivefrom the group consisting of V, Cr, Ti, Ta and Nb or a compound of thesemetals, and at least one metallic binder from the group consisting ofCo, Ni and Fe, at least one of the grain growth-inhibiting additives, atleast locally, having a graduated concentration profile.

[0037] The graduated concentration profile of the graingrowth-inhibiting additive leads to a graduated profile of the carbidegrain size. Consequently, the mechanical properties also have agraduated profile. This is advantageous, for example, where a high wearresistance and bending fracture strength is required at the surface and,at the same time, a high toughness is required in the core, such as forexample in forming tools or tools for diamond production. If theconcentration profile of the gain growth-inhibiting additive is now setin such a way that the concentrations are higher in the regions of theedge zone and decrease in the direction of the center of the component,the edge zone is in fine-grain form, with a graduated transition to themore coarse-grained center. As a result, it is possible to producecomponents with an excellent wear resistance and bending fracturetoughness in the region of the edge zone, in combination with a hightoughness in the center. These components have an improved tool servicelife. A high fracture toughness in the region of the edge zone may alsobe advantageous in the event of a high cyclical or impact shock loading.This is achieved by a reduced grain growth-inhibiting additive contentin the region of the edge zone. The compressive and bending strengthproperties in the core of the component are improved by a graduatedprofile of the grain size and a more fine-grained center. Thisembodiment is also favorable for coated components. The action of theinvention is also achieved if the hard metal alloy contains further,non-carbidic hard material phases, provided that the mechanicalproperties are not significantly adversely affected as a result.

[0038] Advantageous grain growth-inhibiting additives worthy of mentionare vanadium and chromium compounds, the maximum concentration being 2%by weight. Higher contents lead to embrittlement effects. A particularlyadvantageous process is the application of a dispersion or solution tothe surface of a green compact. The dispersion contains the graingrowth-inhibiting additive in extremely finely distributed form. Thegreen compact may be in the as-pressed state. If the green compactcontains additions of wax and/or plasticizer, it may also, according toan advantageous configuration of the present invention, be in thedewaxed or partly dewaxed state. The application of the dispersion orsolution can be carried out, for example, by dipping, spraying orbrushing. The dispersion or solution then penetrates into the interiorof the green compact along open pores. The duration of action and thegrain growth-inhibiting additive content in the dispersion or solutionsubstantially determine the introduction quantity and the penetrationdepth. Therefore, depending on the profile of requirements, it ispossible to set a graduation which extends only on the micrometer scale.However, it is also possible to make the graduation such that it extendsall the way to the center of the component. Furthermore, the process canalso be carried out in such a way that first of all the green compact iscompletely impregnated with the dispersion. This is then removed againfrom the regions close to the surface by means of suitable solvents orby thermal processes. Furthermore, the dispersion may be applied to theentire surface or alternately only to local parts of the surface. Inparticular the local application makes it possible to produce componentsor tools which only have a high hardness where resistance to wear isrequired. The remaining regions have a coarser microstructure with ahigh fracture toughness. Furthermore, it has proven advantageous if thecarbidic component of the green compact has a mean grain size of lessthan 2 μm.

[0039] Other features which are considered as characteristic for theinvention are set forth in the appended claims.

[0040] Although the invention is illustrated and described herein asembodied in a hard metal component with a graduated structure, it isnevertheless not intended to be limited to the details shown, sincevarious modifications and structural changes may be made therein withoutdeparting from the spirit of the invention and within the scope andrange of equivalents of the claims.

[0041] The construction and method of operation of the invention,however, together with additional objects and advantages thereof will bebest understood from the following description of specific embodimentswhen read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0042]FIG. 1 is a graph showing the vanadium content over the specimencross section in a production example according to the invention;

[0043]FIG. 2 is a graph showing the carbide grain size in addition tothe vanadium content, plotted over the cross section;

[0044]FIG. 3 is a graph showing a hardness profile over the specimencross section;

[0045]FIG. 4 is a perspective view of a cross section through a drawingtool; and

[0046]FIG. 5 is a graph showing a hardness profile over the specimencross section.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0047] The following text describes production examples which areintended to explain the implementation of the invention by way ofexample. The figures relate back to the examples 1-3 of the followingdescription.

[0048]FIG. 1 and FIG. 2 relate to example 1; FIG. 3 relates to example2; FIG. 4 and FIG. 5 relate to example 3.

EXAMPLE 1

[0049] A hard metal batch containing 94% by weight of WC with a meangrain size of 1 μm, remainder Co, was produced using the processes whichare standard in the hard metal industry. Green compacts in the shape ofdisposable cutting tool tips were produced by die pressing with apressure of 50 kN. The green compacts were subjected to a standarddewaxing process. Furthermore, a dispersion of distilled water and V₂O₅was prepared, with a solids content of 2% and a mean V₂O₅ particle sizeof less than 50 nm. Then, the green compacts were dipped in theabove-described dispersion for 5 seconds and then dried in air at 50° C.These specimens were sintered in vacuum at a temperature of 1400° C.together with reference green compacts which had not been subjected toany further treatment. The specimens were analyzed by means of electronbeam microprobe, and the microstructural and mechanical characteristicswere determined by a light-microscope examination and hardness testing,in each case on microsections.

[0050]FIG. 1 shows that the vanadium content in the region of the edgezone is 0.24% by weight, and this value decreases gradually toward theinside over the cross section of the specimen. The vanadium content at adistance of 3.8 mm from the specimen edge is 0.08% by weight. In thereference specimen, the corresponding vanadium concentrations were belowthe detection limit of the microprobe. The graduated vanadiumdistribution leads to a graduated grain stabilization effect, asdocumented by the WC grain size values shown in FIG. 2. While the meangrain size increases from the edge zone toward the center, thecorresponding hardness values decrease, as shown in FIG. 3.

EXAMPLE 2

[0051] A hard metal batch containing 89.5% by weight of WC with a meangrain size of 0.8 μm, 0.5% by weight of Cr₃C₂, remainder Co was producedusing the processes which are standard in the hard metal industry. Greencompacts in the shape of disposable cutting tool tips were produced bydie pressing with a pressure of 50 kN. The green compacts were subjectedto a standard dewaxing process. Furthermore, a dispersion of distilledwater and V₂O₅ was prepared, with a solids content of 2% by weight and amean V₂O₅ particle size of less than 50 nm. Then, the green compactswere dipped into the above-described dispersion for 5 seconds and thendried in air at 50° C. These specimens were sintered in vacuum at atemperature of 1400° C. together with reference green compacts which hadnot been subjected to any further treatment. The specimens were analyzedby means of electron beam microprobe, and the microstructural andmechanical characteristics were determined by a light-microscopeexamination and hardness testing.

[0052] The specimens according to the invention once again have agraduated vanadium concentration profile with an edge zone value of0.21% by weight of V and a center value of 0.03% by weight of V. Thecorresponding hardness values are 1 698 HV30 and 1 648 HV30. Thehardness profile is shown in FIG. 3. The reference specimen has ahardness profile which is uniform over the cross section, with a meanvalue of 1605 HV30. The specimens according to the invention and thereference specimens were also subjected to a bending test. The meanobtained from ten measurements is 3950 MPa for the specimens accordingto the invention and 3500 MPa for the comparison specimens.

EXAMPLE 3

[0053] A hard metal batch containing 93.4% by weight of WC with a meangrain size of 2.0 μm, 0.2% of TiC, 0.4% by weight of TaC/NbC, remainderCo was produced using the processes which are standard in the hard metalindustry. Cylindrical green compacts were produced by isostatic pressingat a pressure of 100 MPa and were shaped into a hard metal drawing toolby machining. The green compacts were subjected to a standard dewaxingprocess. Once again, a dispersion of distilled water and V₂O₅ wasproduced, with a solids content of 2% by weight and a particle size ofthe dispersed V₂O₅ particles of less than 50 nm. Then, the dispersionwas applied selectively in the entry and bore region. Drying once againtook place at 50° C. in air. These specimens were sintered in vacuum ata temperature of 1400° C. A microsection was made by metallographicspecimen preparation, as illustrated in FIG. 4.

[0054]FIG. 4 also shows the region where the characterization wasperformed by means of electron beam microprobe and hardness testing. Thevanadium content in the edge zone is 0.18% by weight but is only 0.11%by weight at a distance of 2 mm from the edge of the specimen. FIG. 5shows the gradual hardness profile.

We claim:
 1. An article of manufacture made from a hard metal alloy,comprising: at least one carbide, mixed carbide or carbonitride of themetals selected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V,Nb, Cr, and V; at least one grain growth-inhibiting additive selectedfrom the group consisting of V, Cr, Ti, Ta, and Nb or a compoundthereof, at least one of said grain growth-inhibiting additives, atleast locally, having a graduated concentration profile; and at leastone metallic binder selected from the group consisting of Co, Ni and Fewith a binder content of 0.1-20% by weight.
 2. The article according toclaim 1, wherein, at least locally, the hard metal alloy has a graduatedgrain size profile.
 3. The article according to claim 1, wherein, atleast locally, the hard metal alloy has a graduated hardness profile. 4.The article according to claim 1 formed as a component with an edge zoneand a center, and wherein concentration of said grain growth-inhibitingadditive decreases gradually from the edge zone of the component towardthe center of the component.
 5. The article according to claim 4,wherein the carbide grain size increases gradually from the edge zone ofthe component toward the center of the component.
 6. The articleaccording to claim 1 formed as a component with an edge zone and acenter, and wherein a concentration of said grain growth-inhibitingadditive increases gradually from the edge zone of the component towardthe center of the component.
 7. The article according to claim 6,wherein the carbide grain size decreases gradually from the edge zone ofthe component toward the center of the component.
 8. The articleaccording to claim 1, wherein said grain growth-inhibiting additiveconsists of at least one metal selected from the group consisting of Crand V, or a compound thereof, a maximum content of said graingrowth-inhibiting additive, based on the hard metal alloy, is 2% byweight, and a content thereof decreases gradually to a value x, where0<x<1.0% by weight.
 9. A method of producing the component according toclaim 1, which comprises the following steps: producing a green compactfrom a hard metal alloy, containing at least one carbide, mixed carbideor carbonitride of the metals selected from the group consisting of W,Ti, Ta, Mo, Zr, Hf, V, Nb, Cr, and V, at least one metallic binderselected from the group consisting of Co, Ni, and Fe, an optionaladdition of wax or a plasticizer; producing a dispersion or solutioncontaining at least one grain growth-inhibiting additive selected fromthe group of metals consisting of V, Cr, Ti, Ta, and Nb or a compoundthereof, in finely distributed or dissolved form; applying thedispersion or solution to a surface of the green compact; targetedaction to establish the concentration gradient; and subjecting thearticle to heat consolidation.
 10. The method according to claim 9,wherein the step of producing the green compact comprises compacting ina standard powder metallurgy compacting process or shaping in a standardshaping process.
 11. The method according to claim 9, wherein theapplying step comprises a process selected from the group consisting ofdipping, spraying, and brushing.
 12. The method according to claim 9,wherein the dispersion or solution is applied only to a partial regionof the component surface.
 13. The method according to claim 9, whereinthe carbidic powder component of the green compact has a mean grain sizeof <2 μm.
 14. The method according to claim 9, which comprises at leastpartially dewaxing the green compact by a heat treatment step.
 15. Amethod of producing the component according to claim 1, which comprisesthe following steps: producing a green compact from a hard metal alloy,containing at least one carbide, mixed carbide or carbonitride of themetals selected from the group consisting of W, Ti, Ta, Mo, Zr, Hf, V,Nb, Cr, and V, at least one metallic binder selected from the groupconsisting of Co, Ni, and Fe, an optional addition of wax or aplasticizer; producing a solution containing at least one graingrowth-inhibiting additive selected from the group of metals consistingof V, Cr, Ti, Ta, and Nb or a compound thereof; applying the solution toa surface of the green compact; targeted action to establish theconcentration gradient or complete infiltration; gradually removing thegrain growth inhibitor from regions close to the surface by at least oneof heat treating and dissolving; and subjecting the article to heatconsolidation.
 16. The method according to claim 15, wherein the step ofproducing the green compact comprises compacting in a standard powdermetallurgy compacting process or shaping in a standard shaping process.17. The method according to claim 15, wherein the applying stepcomprises a process selected from the group consisting of dipping,spraying, and brushing.
 18. The method according to claim 15, whereinthe solution is applied only to a partial region of the componentsurface.
 19. The method according to claim 15, wherein the carbidicpowder component of the green compact has a mean grain size of <2 μm.20. The method according to claim 15, which comprises at least partiallydewaxing the green compact by a heat treatment step.